M-Class Dwarfs Could Be Good For Life After All

[Fraser]

SUMMARY: More than half the stars in our galaxy are small, dim M-class stars. Until now, researchers looking for extraterrestrial civilizations have passed over them, since they probably don't give off enough light to support life. But SETI researchers now think that they might be good candidates after all. A planet in orbit around an M-class star would have billions and billions of years orbiting its slow-burning star for life to evolve.

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[John L]

These people always ignore one possible answer to the tidal locking problem. If the Earth-sized world is instead a moon of a gas giant orbiting close in to the M-dwarf star, then the planet will be tidally locked with the star, but the moon will either rotate freely or be tidally locked with the planet. Either way the moon (Earth-sized) will have a normal length day and lots of spectaculat solar eclipses! And the deep red sun sets would be pretty darn romantic, too.

[publiusr]

Wish I could be on such a world.

[GBendt]

Sorry,

if you consider the effect which tidal locking of a planet will have on that planet´s weather system, you will see that due to the tidal locking, the coriolis forces of the air flows will be so small that the planet cannot maintain a wind system which can circle around the planet and carry warmth from the warm side to the cold side.
Thus, the cold side will gradually accumulate an ice shield which will finally gather and contain all the water of the planet. The temperature on the planet´s cold side´s may even fall below a temperature at which the atmosphere´s gases liquifiy, or even freeze to solid "ice", at - 270°C!

Of couse, the cold side of the planet will support a tectonic convection flow from the planet´s hot interior, but this warm mass flow will not deliver enough energy to the cold surface to melt the ice.
If you compare the situation of our earth: The earth´s hot core produces thermal energy of about 10000 Gigawatts, but on the earth´s surface, the energy flow produced by this is only a few milliwatts per square meter. The energy flow which we receive from the sun on that square meter is 18000 times greater!

John, your idea of an earth-type moon orbiting a tidally locked gas giant planet which orbits a red dwarf is tempting, but it is not feasible. A rotating moon-planet system is not able to distribute its common angular momentum such that one orbit of the moon can take a shorter time than the rotation of its planet: If the moon´s orbits takes a shorter time than does the rotation of the planet, tidal forces from the planet will slow the moon´s orbit down. This causes the distance between the two to be reduced, the tidal forces that work on the moon will grow, until both bodies corotate one with the other, both tidally locked to their sun.
And when this sitution is reached, this moon will experience the same fate than any tidally locked body which orbits a sun closely...

The close red sun will be grilling the sides of the moon and of the planet which lie in a never ending day. At the same time, the dark sides of both lie in a permanent night of utmost frostiness.

A dark night, crystal clear and full of stars, at excellent seeing conditions, and no one there to watch it...

Regards,

Günther

[John M. Dollan]

There used to be some papers available online, perhaps around ten years old now, demonstrating that the heating effect from the red dwarf would be quite sufficient to keep the atmosphere in continual crculation. Unfortunately, I can't find them. Anyone have a clue where to look? I've tried everything from Google to direct emailing a couple of universities.

...John...

[wstevenbrown]

A Quibble

Certainly one may obtain comfortable heat from a dim star by choosing an orbit closer in. BUT the flares of M-class stars are comparable in size and energy to those on F and G stars. Expressed as a fraction of the M star's output, they can cause variations of 10% to 300%. Now that's what I call weather! (To be fair, not all M stars are that truculent-- it seems to correlate with metallicity/composition, but there is no consensus.) S

[IsaacKuo]

About tidal locking--why is this a given? For a long time, scientists suspected Mercury to be tidal locked to the Sun, but it turned out that it isn't. Instead, it rotates with respect to the Sun in a self-reinforcing resonance with its (very slightly) elliptical orbit. Why couldn't planets around M-class stars have similar rotation?

Anyway, I speculate that an advanced interstellar civilization might actually be more interested in living around a flare star itself than on any planets around it. They could live in orbit around the star, harvesting raw materials from the frequent flares.

[eburacum45]

The article suggests that older M-class dwarfs are less prone to flares.

Scientists have also learned that most of an M dwarf's hyperactivity occurs early in its life cycle, during the first billion years or so. After that, the star tends to settle down and burn quietly for many billions of years more. Once the fireworks end, life might be able to take hold.

So older M class stars might be good for life.

A planet rotating in harmonic resonance with its orbit, like Mercury, is also possible; such a planet might have an orbital period of about thirty days or so in the habitable zone, and that would lead to a daylength of about ten days. Not very Earth-like, but would make an interesting planet for a fictional treatment. Over time I think that even a harmonically locked planet will become completely locked, especially one with a ocean layer which I think would increase tidal braking.

[GBendt]

Hi,

Red dwarfs have a low mass and produce rather little energy compared to the energy output of our sun. As there is a relation of mass to luminosity which says that the luminosity of a star rises by the third power of its mass, a red dwarf of 10% of the solar mass thus produces 1/1000th of the energy of our sun. To be within the habitable zone of that red dwarf, a planet orbiting this red dwarf must orbit at a radius which is the earth´s orbit radius divided by the square root of 1000.
The resulting radius is 4,7 million kilometers. The orbital speed is 57 km/s, and it will take the planet 5,1 days to complete an orbit.

To be able to hold an atmosphere, the planet should have about the mass of the earth. If the planet´s mass is lower, its atmosphere would gradually get lost in space. Liquid water cannot exist on a planet on which air pressure is too low.

If the planet´s mass is higher, the planet will hold a denser atmosphere, and a denser atmosphere will create higher ground temperatures and dense clouds (think of venus ...).

The planet will orbit the red dwarf gravity-locked. Why?
Consider the planet rotating while it orbits the red dwarf in the habitable zone around the red dwarf, at an orbit radius of 4,7 million kilometers. What happens?
The gravity pull by the red dwarf which is faced by a spot on the rotating planet is different, depending on whether from this spot´s view the red dwarf is seen at the zenit or at the nadir. When the red dwarf is at the zenit, this spot is closer to the red dwarf. The gravity pull then is a bit stronger than it is for this same spot when this spot has moved to where the red dwarf is at the nadir. There, the distance from the red dwarf is greatest, and thus the gravity pull by the red dwarf is a bit less. For a planet of earth´s size at an orbit of 4,7 million kilometers, the difference is a mere 7e-6, but this difference will force the planet into a gravity locked rotation after millions of years.

All you need to do is apply simple physics if you want to understand what happens. It´s nice to imagine how it would look like if a big red sun would be shining above a weird horizon, but imagination is best based on facts.

Neither Mercury nor Venus orbit within the habitable zone provided by our sun.
If Mercury would orbit a red dwarf, its orbit would be much different from the orbit it has around our sun.

Regards,

Günther

[Doodler]

Why would planets orbiting red dwarves be any more likely to be tidally locked than planets orbiting other stars? If you had one like Earth, with a fairly sizeable satellite orbiting it, why wouldn't it be able to retain rotational momentum for an extended period of time?

[Van Rijn]

Why would planets orbiting red dwarves be any more likely to be tidally locked than planets orbiting other stars? If you had one like Earth, with a fairly sizeable satellite orbiting it, why wouldn't it be able to retain rotational momentum for an extended period of time?

The issue is that M-class dwarf stars are very dim. A planet in the "habitable zone" (not cold by earth standards) would have to be very close to the star, where it would become tidally locked fairly quickly. A more distant and cold planet would not.

If it is a moon of a gas giant, or a double planet, that would change the rules somewhat.

[eburacum45]

Hi,

Red dwarfs have a low mass and produce rather little energy compared to the energy output of our sun. As there is a relation of mass to luminosity which says that the luminosity of a star rises by the third power of its mass, a red dwarf of 10% of the solar mass thus produces 1/1000th of the energy of our sun. To be within the habitable zone of that red dwarf, a planet orbiting this red dwarf must orbit at a radius which is the earth´s orbit radius divided by the square root of 1000.
The resulting radius is 4,7 million kilometers. The orbital speed is 57 km/s, and it will take the planet 5,1 days to complete an orbit.

Agreed; but not all red dwarfs are as small as that. Some are big enough to make it into Tarter and Turnbull's catalogue of habitable stars. If a star has a luminosity 0.05 times that of the Sun, it will be a red dwarf , but the habitable zone will be twenty million miles from the star. Such a planet could easily have a harmonic rotation period rather than be tidally locked.

However this might not be neccessary; a tidally locked planet might even be habitable, especially if the simulation used by the National Geographic in this program is anything like correct. Circulation of the atmosphere occurs, leading to a permanent cyclone at the sub-stellar point, and apparently enough mixing to maintain habitable temperatures all over one side of the world.

Alternatively there is another possibility for habitable tidally locked worlds; I have used an older model in this fictional planet. The substellar point is hot desert, and the dark side is frozen; but at the terminator there is a small temperate zone, with some small variation in temperature because of libration. This older model might also allow limited biospheres to develop even on planets orbiting around smaller red dwarfs.

[IsaacKuo]

One thing I don't understand about the "habitability zone"...life as we know it on Earth requires liquid water and...that's about it. Liquid water. Since the concept of "habitability zone" was invented, we've discovered life on Earth in places of all extremes of temperature and pressure so long as liquid water is present.

So, what's wrong with a Venus-like planet with a superthick atmosphere? Even if the pressure at the surface is high, as long as it's the right temperature for liquid water there should be the potential for Earth-like life. With a thick atmosphere, even a tidally locked planet would have a stable temperature at the surface.

[galacsi]

Tidally locked planet but with a libration zone, Big moon orbiting a giant planet, Planet rotating in harmonic resonance with its orbit , Double planet and now planet with a dense atmosphere !

So there is plenty of possibilities for life to prosper arount a M dwarf.
An other reason to think life must be frequent in our galaxy.

[Thanatos]

Also consider a planet with a dense atmosphere, or predominantly oceanic surface might take a very long time to become tidally locked within the habitable zone of an M class star. I would also think there would be enough 'wobble' to ensure a planetary atmosphere would always circulate to some extent. Furthermore, even a tidally locked planet might easily have a twilight zone where life could florish. Life, by definition, is a survivor and very stubborn by nature.

[galacsi]

eburacum45

I could not reach the simulation used by the National Geographic in this program

But your own model i could and found it very interesting. fictional planet.

[galacsi]

I thought of an other possibility for a planet , tidally locked or out of the life zone to receive enough light and warmth to become habitable.

The star and the planet around is inside a dust cloud or surrounded by a ring of dust. indirect lightning , very great view. But i don't know if this situation can last a sufficient time.